Gas turbine engines have combustion chambers within which fuel is burnt to heat a working fluid. The temperature within a combustor can be as high as 1900 K. Fuel is supplied through fuel injectors that are specifically designed to withstand high temperatures and effectively atomise the fuel.
In some fuel injectors, particularly those that are known as “dual fuel” injectors, the fuel injector requires an internal fuel tube that isolates a first form of fuel from a second form of fuel. The first form of fuel is typically a liquid and the second form of fuel is typically a gas, though both may be liquids or gasses of the same or differing composition depending on the design and required energy output from the gas turbine.
The fuel injectors have significant temperature variations in operation. At start up the temperature can be close to ambient whilst at operating conditions where power produced by the engine is at a maximum the temperature can be in excess of 1000K. Thermal expansion of the tubes and the injector housing create stresses within the components that can cause fatigue and damage to the tube or housing. The stresses are exacerbated by the temperature difference between the relatively cold fuel in the tubes and the relatively hot housing. The fuel remains at a relatively constant temperature that is approximately equal to the fuel inlet temperature and remains within approximately 30 degrees Celsius at the point of injection regardless of the power output of the engine.
In a known injector, described with reference to FIG. 1 the injector housing comprises two pieces 102,106 jointed by a weld joint. The first piece 102 comprises the injector stalk 104 that leads to the injector head (not shown). The second piece 106 incorporates the connector 108 into which the injector fuel tube 110 fits and also comprises a port 112 into which an engine fuel tube is inserted.
Fuel is supplied to the injector head from the engine fuel tube (not shown) inserted into the top port 112 of the housing. The fuel flows through a weight type distributor 116 positioned in the second piece of the housing and into the fuel tube 110.
The connector 108 is angled such that its axis 122 is aligned with the expansion axis of the fuel pipe 110. Leakage of fuel is prevented by providing dynamic seals between the connector 108 and the fuel tube 110. The seals allow limited relative movement between the two components caused by operational temperature differences of the relatively cool fuel pipe 110 and the relatively hot housing 102, 106.
The housing is assembled by placing the second housing piece over the first housing piece such that the connector engages and seals with the injector fuel tube. Because the connector 108 is provided at an angle to the main axis 120 of the second component it is not possible to rotate the first piece of the housing 102 relative to the second piece 106. Consequently, the two housing pieces 102,106 are then secured together by a weld joint. The joint creates a fluid tight chamber 114 isolated from the internal liquid fuel passage and into which, in operation, a gaseous fuel may be supplied and fed to the injector head.
A weld joint offers a number of advantages: it is light and has high integrity. Failure of an internal seal therefore does not result in an overboard leak as the leaking fuel is retained within the housing and fed by the inner tubes to the combustor.
The weld joint is considered to be permanent as it can only be broken by cutting the joint and then reforming the weld. Consequently, it is expensive and time consuming to inspect and replace any of the internal seals. Additionally, the weld joint requires an internal braze the quality of which it is difficult to assess. It is difficult to heat treat the weld between the two housing pieces without damaging the inner seals.
It is an object of the present invention to seek to provide an improved injector assembly that addresses these and other problems.